According to the literature, urokinase-type plasminogen activator (u-PA) has been found in all mammalian species so far investigated. Several findings relate u-PA to tissue degradation and/or cell migration, presumably through a breakdown of the extracellular matrix, caused by plasmin together with other proteolytic enzymes. This relation has been most extensively studied in postlactational involution of the mammary and prostate gland and the early phase of trophoblast invasion after implantation of the fertilized egg in the uterus. The hypothesis of a role of u-PA in tissue degradation and cell migration is further supported by the more exact localization made possible by the immunocytochemical findings of u-PA in epithelial cells of involuting mammary glands, in areas with tissue degradation in psoriasis, in association with the release of spermatocytes during spermatogenesis, and in keratinocytes of the epithelial outgrowth during wound healing (see Dano et al., 1988, 1990, Grondal-Hansen et al., 1988, Andreasen et al, 1990).
It is also conceivable that u-PA plays a role in the degradative phase of inflammation, and there have also been reports that u-PA interferes with the lymphocyte-mediated cytotoxicity against a variety of cells, and a direct role of u-PA in the cytotoxic effect of natural killer cells has been proposed. A role of u-PA has been proposed in angiogenesis and in endothelial cell migration, a process important in tumor growth.
u-PA is produced by many cultured cell types of neoplastic origin. It has been found that explants of tumor tissue released more u-PA than the corresponding normal tissue. u-PA has been identified in extracts from human lung, colon, endometrial, breast, prostate and renal carcinomas, human melanomas, murine mammary tumors, the murine Lewis lung tumor, and in ascites from human peritoneal carcinomatosis. An immunohistochemical study of invasively growing and metastasing Lewis lung carcinomas in mice consistently showed the presence of u-PA, but also a pronounced heterogenecity in the content of u-PA in different parts of the individual tumors. A high u-PA content was found in areas with invasive growth and degradation of surrounding normal tissue, while other areas were devoid of detectable u-PA. The u-PA was located in the cytoplasm of the tumor cells and extracellularly surrounding the tumor cells.
Degradation of the surrounding normal tissue is a central feature of invasiveness of malignant tumors. The constant finding of u-PA in malignant tumors and the findings indicating that u-PA plays a role in tissue degradation in normal physiological events have led to the assumption that u-PA plays a similar role in cancer development. The hypothesis of u-PA playing a role in tissue destruction involves the assumption that plasmin, together with other proteolytic enzymes, degrades the extracellular matrix. It is noteworthy in this context that most components of the extracellular matrix can be degraded by plasmin. These include laminin, fibronectin, proteoglycans, and possibly some types of collagen, but not all. In addition, as originally reported by Vaes and collaborators, plasmin can activate latent collagenases which in turn can degrade the other types of collagen (see Dano et al., 1988, 1990).
The majority of the cancer patients in the treatment failure group succumb to the direct effects of the metastases or to complications associated with the treatment of metastases. Therefore, much research has been focused on identifying specific biochemical factors which can be the basis for diagnostic or therapeutic strategies. The extracellular matrix is composed of glycoproteins such as fibronectin and laminin, collagen and proteoglycans. Extracellular matrix becomes focally permeable to cell movement only during tissue healing and remodelling, inflammation, and neoplasia. Liotta (1986) has proposed a three-step hypothesis: The first step is tumor cell attachment via cell surface receptors. The anchored tumor cell next secretes hydrolytic enzymes (or induces host cells to secrete enzymes) which can degrade the matrix locally (including degradation of the attachment components). Matrix lysis most probably takes place in a highly localized region close to the tumor cell surface. The third step is tumor cell locomotion into the region of the matrix modified by proteolysis. Thus, invasion of the matrix is not merely due to passive growth pressure but requires active biochemical mechanisms.
Many research groups have proposed that invasive tumor cells secrete matrix-degrading proteinases. A cascade of proteases including serine proteases and thiol proteases all contribute to facilitating tumor invasion. One of the crucial cascades is the plasminogen activation system. Regulation of the proteolysis can take place at many levels including tumor cell-host cell interactions and protease inhibitors produced by the host or by the tumor cells themselves. Expression of matrix-degrading enzymes is not tumor cell specific. The actively invading tumor cells may merely respond to different regulatory signals compared to their non-invasive counterparts (Liotta, 1986).
The assumption that the plasminogen activation system, through a breakdown of extracellular matrix proteins, plays a role in invasiveness and destruction of normal tissue during growth of malignant tumors is supported by a variety of findings. These include a close correlation between transformation of cells with oncogenic viruses and synthesis of u-PA, the finding that u-PA is involved in tissue destruction in many non-malignant conditions, and the immunohistochemical localization of u-PA in invading areas of tumors (see Dano et al., 1985, Saksela, 1985, for reviews).
Further support for this hypothesis has come from studies with anti-catalytic antibodies to u-PA in model systems for invasion and metastasis. Such antibodies were found to decrease metastasis to the lung from a human u-PA producing tumor, HEp-3, transplanted onto the chorioallantoic membrane of chicken embryos (Ossowski and Reich, 1983, Ossowski 1988), penetration of amniotic membranes by B16 melanoma cells (Mignatti et al., 1986), basement membrane invasion by several human and murine cell lines of neoplastic origin (Reich et al., 1988), and formation of lung metastasis after intravenous injection of B16 melanoma cells in mice (Hearing et al., 1988). In some of these studies (Mignatti et al., 1986, Reich et al., 1988), a plasmin-catalyzed activation of procollagenases (see Tryggvason et al., 1987) appeared to be a crucial part of the effect of plasminogen activation.
A requirement for the regulation of a proteolytic cascade system in extracellular processes is the precise localization of its initiation and progression. For example, in the complement and coagulation systems, cellular receptors for various components are known and serve to localize reactions that either promote or terminate the reaction sequence (Muller-Eberhard, 1988, Mann et al., 1988). In the plasminogen activation system, the role of fibrin in the localization of plasminogen activation catalyzed by the tissue-type plasminogen activator (t-PA) is well known (Thorsen et al., 1972, Hoylaerts et al., 1982).
Immunocytochemical studies have suggested that in the invasive areas of tumors, u-PA is located at the membrane of the tumor cells (Skriver et al., 1984), and recent findings indicate that at cell surfaces, u-PA is generally bound to a specific receptor and that this localization may be crucial for the regulation of u-PA catalyzed plasminogen activation in time and space (see Blasi et al., 1987, Dano et al, 1990). Preliminary reports suggest that also t-PA may bind to cell surface receptors and retain its enzymatic activity (Beebe, 1987, Barnathan et al., 1988, Hajjar and Nachmann, 1988, Kuiper et al., 1988). This phenomenon, however, awaits further clarification concerning the nature of the binding sites.